The spring rates on aftermarket valves are typically firmer then that of an OEM valve.
The OEM valve will be looser and may even remain slightly open at idle (vacuum). This causes un-metered air to escape, resulting in erratic idle.
I recommend (even if you are not currently experiencing an issue) that you contact GReddy or an authorized distributor to purchase a BOV outlet of the correct diameter.
http://www.greddy.com/products/displ...SubCategory=18
There are multiple reasons why you don't want to VTA on a MAF car. It's only going to cause issues in the long run.